Modular basal thumb joint implant

ABSTRACT

Basal thumb joint implant has a head with a smooth, generally hemispherical, medio-proximally directed, articulating surface, and a generally abrupt, distally directed, truncation thereto; and a stem attached to the head, which arises from the truncation of the head and includes at least one of the following features:
         A) a general angle of attachment to the head which is acute in relation to the truncation of the head;   B) a flanged cross-sectional stem profile;   C) an inwardly curved stem;   D) an eccentric head attachment site for the stem.
 
The implant may be one-piece or modular in construction. The modular basal thumb joint implant, however, which is not necessarily limited by requiring inclusion of the aforesaid additional features A-D, includes a head with a smooth, generally hemispherical, medio-proximally directed, articulating surface, a generally abrupt, distally directed truncation thereto, and a stem trunion-receiving cup in the truncation; and a stem attachable to the head, which stem has intracarpal spike-like distal end, and a proximally directed trunion, which trunion is insertable into the stem trunion-receiving cup of the head. Other digits may be provided with analogous implants, particularly with respect to those digital implants amenable to modularity.

This is a divisional of application Ser. No. 09/352,472 filed on Jul.14, 1999 A.D.

BACKGROUND TO THE INVENTION

I. Field and Purview of the Invention

The present invention concerns in particular a one-piece or a modularbasal thumb joint implant for the trapeziometacarpal joint, and can alsoinclude an extension of the concept of modularity to other digitaljoints. The implant generally includes head and stem portions, and ithas one or more of the following: a generally acute head-stem attachmentorientation; a flanged cross-sectional stem profile; an inwardly curvedstem; and an eccentric head attachment site for the stem.

II. Prior Art, and Discovered Problems

Cases of degenerative or post-traumatic arthritis of thetrapeziometacarpal joint are known to leave the joint unstable,disfunctional, and painful. In addressing this problem, the SwansonBasal Thumb Joint was developed for use as an adjunct to resectionarthroplasty of the joint. The Swanson implant has a generally simple,linearly straight, non-curved stem, which is squarelike incross-section; when fitted into the intramedullary canal of the firstmetacarpal bone it is reportedly designed to resist rotation. It alsohas a convex head, which fits into a jointlike concave surface fashionedin the opposing distal part of the trapezium bone and which reportedlyhelps restore joint stability and motion. The Swanson joint is aone-piece unit made from unalloyed titanium (ASTM F67). See, the WrightMedical Technology brochure, “Swanson Titanium Basal Thumb Implant.”

Problems with the foregoing, however, have been discovered to include 1)the straight, non-anatomical countour of the stem, which (a) reduces therotational stability of the implant, and (b) induces outward mechanicalleveraging of the prosthetic head and the proclivity for lateraldislocation of the replaced joint; 2) the demonstrated inferiortribological characteristics of an articulating titanium surface; and 3)the one-piece construction or non-modularity of the implant, whichprecludes an ability to “mix and match” component parts, consequentlyrequiring a redundant and costly on-the-shelf inventory of implants toassure the availability of an implant which will provide an appropriatehead-stem combination that will produce a precise, individualizeddimensional fit of both the head and the intramedullary stem parts ofthe prosthesis. Also, 4) the square-like configuration of the stem,which is the stabilizing part of the composite implant, provides a grossmisfit with the curved and elliptically rounded inner counter of thenormal medullary canal, which, as a consequence, requires excessiveresectional depletion of endosteal bone to obtain an adequately intimatebone-stem interface fit to assure the long term stability of theimplant.

In the hip joint implant field, it is known to employ certain principlesof modularity. Certain appropriately sized heads that mate with theacetabular socket may be interchanged for assembly with certain properlysized stems of the femoral component.

SUMMARY OF THE INVENTION

The present invention provides, in one aspect, a basal thumb jointimplant comprising a head including a smooth, generally hemispherical,medio-proximally directed, articulating surface, and a generally abrupt,distally directed, truncation thereto; and a stem attached to the head,which arises from the truncation of the head and includes at least oneof the following features:

-   -   A) a general angle of attachment to the head which is acute in        relation to the truncation of the head;    -   B) a flanged cross-sectional stem profile;    -   C) an inwardly curved stem;    -   D) an eccentric head attachment site for the stem.        The implant may be one-piece or modular in construction. The        modular basal thumb joint implant, however, which is not        necessarily limited by requiring inclusion of the aforesaid        additional features A-D, includes a head with a smooth,        generally hemispherical, medio-proximally directed, articulating        surface, a generally abrupt, distally directed truncation        thereto, and a stem trunion-receiving cup in the truncation; and        a stem attachable to the head, which stem has intracarpal        spike-like distal end, and a proximally directed trunion, which        trunion is insertable into the stem trunion-receiving cup of the        head. Other digits may be provided with analogous implants,        particularly with respect to those digital implants amenable to        modularity.

The invention is useful in digital arthroplasty.

Significantly, by the invention, problems in the art are ameliorated ifnot overcome. The inward (varus) curve of the stem proximally and theeccentric medial placement of the head on the stem avoid the propensityfor dislocation of the replaced joint. This anatomically orientedarrangement also permits an unobstructed range of normal pain freemotion. The anatomic stem curvature in conjunction with the flangedcross-sectional stem profile, which preferably is a tri-flangedcross-sectional stem profile, provides for a more precise fit with themetacarpal medullary canal anatomy, hence preserving bone stock andassuring optimal long term stability, including near if not completeimmovability with respect to rotation, of the implant. The “mix andmatch” modularity of variably sized heads and stems allows for selectiveassembly of a composite implant which 1) provides a precise fit for bothparts, head and stem, and 2) reduces the on-the-shelf inventory of thecomposite implant, which, in turn, reduces the cost of the procedure.

Numerous further advantages attend the invention.

DRAWINGS IN BRIEF

The drawings form part of the specification hereof. With respect to thedrawings, the following is briefly noted:

FIG. 1 is a medial-lateral view of a basal thumb joint implant of theinvention.

FIG. 2 is a medial view of the implant of FIG. 1.

FIG. 3 is a lateral view of the implant of FIG. 1.

FIG. 4 is a medial-lateral view of a modular basal thumb joint implantof the invention, otherwise generally analogous to an implant such as inFIGS. 1-3, and with its head in section.

FIG. 5 is a sectional view of the stem of the implant of FIG. 4, takenalong 5-5 in FIG. 4.

FIG. 6 is a sectional view of the stem of the implant of FIG. 4, takenalong 6-6 in FIG. 4.

FIG. 7 is a medial-lateral view of the stem of the implant of FIG. 4.

FIG. 8 is a lateral view of the stem of FIG. 7.

FIG. 9 is a medial-lateral view of the head of the implant of FIG. 4.

FIG. 10 is a medial-lateral view of a basal thumb joint implant of theinvention, otherwise as of FIG. 1 or 4, with a pore-coat thereon.

FIG. 11 is a distal view of the implant of FIG. 10.

FIG. 12 is a view, generally from a medial-lateral Perspective, of abasal thumb joint implant of the invention, in a 14.5-millimeter(14.5-mm) finished demonstration model size, with an inwardly curvedstem and a more eccentric attachment situs for the stem than would bedepicted for the implants of FIGS. 1, 4 & 10, with a rule in inchesalongside.

FIG. 13 is a sectional view of the basal thumb joint implant such as ofFIGS. 1, 4, 10 and/or 12 in place in the hand.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THE INVENTION

The invention can be further understood by the present detail, which maybe read in view of the drawings. Such is to be taken in an illustrative,and not necessarily limiting, sense.

In general, the joint implant of the invention includes a head and astem. It may be one-piece or modular in construction.

The implant can be made of any suitable material to includebiocompatible ceramics, metals, plastics or other suitable material.However, manufacture from a cobalt-containing alloy is beneficiallyemployed since, among other considerations, not the least of which isrelative ease of manufacture, a better, more triboligically efficientarticular surface can be provided than from the softer titanium,particularly when the implant is of one-piece construction. In modulardigital joint implants, the cobalt-containing alloy may be employed forboth head and stem components, or, preferably, the cobalt-containing, orother, alloy is employed to make the stem component, and a ceramic isemployed to make the head component since, among other considerations,the ceramic can make for an even more smooth articular surface. Forexample, the implant of the invention which is one-piece or is modularcan be machined from a cobalt-containing alloy made in accordance withASTM F75, or even in accordance with ASTM F799 or ASTM F1537, orfurther, the modular implants of the invention can have its stemmachined from the ASTM F75 cobalt-containing alloy and its head machinedfrom, ZIRALLOY ceramic. The head, especially its smooth articulatingsurface, may be made from another hard, smooth material.

With respect to the drawings, in FIGS. 1-3, a one-piece implant isdepicted; in FIGS. 4-9, modularity is introduced; FIGS. 10 & 11 showpore-coating of an implant; FIG. 12 shows more eccentricity andcurvature with respect to the stem, and FIG. 13 generally shows a basalthumb joint implant of the invention implanted in the hand. In general,basal thumb joint implant 100 for mounting in and articulating with aprepared surface of suitable bone stock 9 includes trapezium-mating head10 and intrametacarpal stem 20.

The head 10 generally includes articulating surface 11, which is smooth,say, to a Number-4 (⁴√{square root over ( )}) tolerance, for example,when made of suitable metal, or better, say, if made of suitable ceramicor other suitable material, is generally hemispherical and is proximallydirected, and includes truncation 12, which is generally abrupt anddistally directed, and may take the form of a flat plane. See, FIGS. 1,3, 4, 9-13. The truncation 12 may be roughened, say, if of suitablemetal as by sand-blasting with glass bead BT-Number-12. In addition toor in lieu of the sand-blasting, porous coating 13 may be present, forexample, a 0.020-inch porous coating under the head 10 and around partof the stem 20 near the head, say, for a 0.140-inch distance 14, of thecobalt-chromium-molybdenum powder type to ASTM F-1377-92 specificationsmay be applied to the implant made from the cobalt-containing alloy.See, FIGS. 10 & 11. The convex articulating surface 11 can register andarticulate with a correspondingly concavely prepared part of the distaltrapezium bone; the truncation 12 can mate with and reside on aproximally resected mesa of the first metacarpal bone. See, FIG. 13.

In modular joint implants, the head 10 can also include trunionreceiving cup 15 which may be, say, cylindrical, conical, frustoconical,or have an elliptical, chordated curvilinear, triangular, rectangular,square, or other cross-section. The cup 15 can include walls 16, whichmay be tapered as for a Morse taper, and base 17.

The stem 20 generally includes intramedullary spike 21 may taper fromthe head 10 to its end distal from the head 10. See, FIGS. 1-8 & 10-13.Thus, among other advantages, the implant 100 may be pressed into placein the metacarpal bone. The stem 20 may be roughened, say, as bysand-blasting with glass bead BT-Number-12. In addition to or in lieu ofthe sand-blasting, the pore-coating 13 may be applied, especially aboutthe more proximally directed portions of the stem 20. See, FIGS. 10 &11. The stem/spike 20/21 may have a general angle 22 of attachment tothe head which is acute, for instance, about from sixty-five toseventy-five, say, about seventy, degrees in relation to the truncation12 of the head 10, which may yield complementary angle 22 c. Thestem/spike 20/21 may have a flanged cross-sectional stem profile. Forexample, the flanged stem profile may have a tri-flanged cross sectionsuch as provided by being generally T-shaped, especially about its moreproximal positions. See, e.g., FIG. 5. The flanged stem profile maytaper to a more generally triagonally shaped cross-section, especiallyabout its more distal positions. See, e.g., FIG. 6. Accordingly, thespike 21, which may be termed an intracarpal spike-like stem orappendage, can taper toward its distal end, for example, at half angle21 a as follows: 13-mm and 14-mm sizes, 10-degree half angle; 16-mmsize, 12-degree half angle; 17.5-mm and 19-mm sizes, 13-degree halfangle. Thus, not only can the stem 20 be readily inserted into thedigital bone stock which can be more easily prepared to receive such ashape and conserve bone in the procedure but also the flanges can morestrongly hold the stem in the bone. The stem/spike 20/21 may be inwardlycurved with concave 23 and/or convex 24 components, and/or have aneccentric attachment site, offset from the center of the head 10. See,e.g., FIGS. 1, 4, 7, 10 & 12. Preferably, however, the basal thumbimplant 100 generally includes all of such features. Thus, anatomicalcooperation and normal functioning can better ensue.

With the modular basal thumb joint implant 100, the stem is attachableto the head. Its stem 20 can have not only the intracarpal spike-likeappendage 21, which, again, may be tapered toward its distal end, butalso a proximally directed trunion 25, which itself may be, say,cylindrical, conical, frustoconical, or have an elliptical, chordatedcurvilinear, triangular, rectangular, square, or other cross-section.The trunion 25 can include walls 26, which may be tapered as for theMorse taper, and cap 27. The trunion 25 is insertable into the stemtrunion receiving cup 15 of the modular head 10, and, in general, itswalls 26 and cap 27 suitably correspond to the walls 16 and base 17 ofthe modular head 10. See, FIGS. 4, 7-9.

Actual dimensions of the implant may vary according to needs or desires.For example, basal thumb joint implants 100 can have for its head 10with noted diameter 30 (size in millimeters (mm)) the dimensions (listedin inches) which follow:

13-mm 14.5-mm 16-mm 17.5-mm 19-mm Spherical radius 31 0.250 0.281 0.3130.344 0.375 (FIG. 1) Spherical radius 31 0.250 0.281 0.312 0.344 0.375(FIG. 9) Head height 32 0.289 0.320 0.373 0.413 0.455 (FIG. 1) Headheight 32 0.290 0.331 0.372 0.414 0.455 (FIG. 9) Distance 33 as cast0.170 0.200 0.230 0.225 0.250. (FIG. 1)In addition, the modular head 10 can have its stem trunion receiving cup15 include the following dimensions (FIG. 9): base diameter 34, 0.1514″;wall-base radius 35, 0.015″; cup depth 36, 0.150″; wall half angle taper37, 2.8625-degrees. Also, with respect to FIGS. 1 & 9, the heads 10 canhave a 0.020-inch (″) truncation radius 38. As well, the dimensions ofthe stem 20 can vary as necessary or desired, for example, referring toFIG. 1, radius 40 can be for the 13-mm size implant, 0.252″; 14.5-mm and16-mm, 0.315″; 17.5-mm and 19-mm, 0.394″; referring to FIGS. 5-8, thefollowing dimensions (in inches) may obtain:

Small Medium Large Radius 40 (FIG. 7) 0.2522 0.3152 0.394 Radius 50(FIG. 5) 0.0145 0.0181 0.0226 Radius 51 (FIG. 5) 0.0198 0.0248 0.031Radius 52 (FIG. 5) 0.0282 0.0352 0.044 Length 53 (FIG. 5) 0.0554 0.06930.0866 Length 54 (FIG. 5) 0.2522 0.3152 0.394 Width 55 (FIG. 5) 0.03990.0499 0.0624 Width 56 (FIG. 5) 0.2522 0.3152 0.394 Width 60 (FIG. 6)0.1022 0.1246 0.1557 Width 61 (FIG. 6) 0.0162 0.0197 0.0246 Length 62(FIG. 6) 0.0201 0.0245 0.0306 Length 63 (FIG. 6) 0.1022 0.1247 0.1557Radius 64 (FIG. 6) 0.0114 0.0139 0.0174 Radius 65 (FIG. 6) 0.0058 0.00710.0089 Height 70 (FIGS. 4,7) 0.3547 0.4434 0.5542 Width 71 (FIG. 7)0.2522 0.3152 0.394 Width 72 (FIG. 7) 0.2370 0.2962 0.3702 Width 73(FIG. 7) 0.0997 0.1246 0.1557 Radius 74 (FIG. 7) 0.5659 0.7074 0.8842Radius 75 (FIG. 7) 0.0498 0.0623 0.0799 Radius 76 (FIG. 7) 0.2522 0.31520.394 Length 80 (FIG. 8) 0.2522 0.3152 0.394 Height 81 (FIG. 8) 0.08620.1078 0.13475 Height 82 (FIG. 8) 0.5165 0.6456 0.807 Radius 83 (FIG. 8)0.0416 0.052 0.065.In addition, there may be 11-degree angle 57 and 170-degree angle 58(FIG. 5); 62-degree angle 66 (FIG. 6); 0.151-inch (″) trunion referencediameter 77 and trunion height 78 of 0.125″ for the trunion 25, whichmay have a 0.010″×45-degree circumferential top chamfer 79 and a2-degree 47-minute 45-second half angle taper to the wall 26 (FIG. 7);and a 12-degree angle 84 (FIG. 8).

The implant is implanted at the discretion of the surgeon. Surgicalcement such as polymethylmethacrylate may be employed.

CONCLUSION

The present invention is thus provided. Various features,subcombinations and combinations can be practiced with or withoutreference to other features, subcombinations or combinations in thepractice of the invention, and numerous adaptations and modificationscan be effected within its spirit, the literal claim scope of which isparticularly pointed out as follows:

1. A modular basal thumb joint implant comprising the following parts: ahead including a single, smooth, generally hemispherical,medio-proximally directed, articulating surface, and a generally abrupt,distally directed, planar end to the head which defines an end to saidarticulating surface and has a center, said articulating surface beingcontinuous as to its sphericity and uninterrupted up to the end of saidarticulating surface so that said articulating surface defines atruncated ball of a shape that is from substantially hemispherical togreater than substantially hemispherical; and a stem, which isattachable to the head, and which, when attached to the head, projectsfrom the head along an axis, which arises from the generally planar endto the head and includes at least one of the following features: A) ageneral angle of projection from the head that is acute in relation tothe generally planar end to the head; B) a flanged cross-sectional stemprofile, which, when taken in cross-section perpendicularly to the stem,is in a tri-flange shape, with three flanges without notches extendingdistally on the stem; C) an inwardly curved stem; D) an eccentric headsite for the stem, which is offset from the center of the generallyplanar end of the head; wherein said implant has its head of a size formounting in and articulating with a correspondingly concavely preparedsurface of trapezium bone stock, and its stem of a size forintramedullary insertion in metacarpal bone stock.
 2. The implant ofclaim 1, which has at least the general angle of projection from thehead which is acute in relation to the generally planar end to the head.3. The implant of claim 2, which further includes at least one of theflanged cross-sectional stem profile, the inwardly curved stem, and theeccentric head site for the stem.
 4. The implant of claim 2, whichfurther includes the flanged cross-sectional stem profile, the inwardlycurved stem, and the eccentric head site for the stem.
 5. The implant ofclaim 4, wherein the head has a stem trunion receiving cup in thegenerally planar end to the head, and the stem has a trunion for beingreceived in said cup.
 6. The implant of claim 5, which has tapered wallsto said cup and said trunion for securing the head and stem together. 7.The implant of claim 2, wherein the head has a stem trunion receivingcup in the generally planar end to the head, and the stem has a trunionfor being received in said cup.
 8. The implant of claim 7, which hastapered walls to said cup and said trunion for securing the head andstem together.
 9. The implant of claim 2, wherein the head is made of asuitable ceramic material, and the stem of a suitable metal.
 10. Theimplant of claim 1, which has at least the flanged cross-sectional stemprofile.
 11. The implant of claim 1, which has at least the inwardlycurved stem.
 12. The implant of claim 1, which has at least theeccentric head site for the stem.
 13. The implant of claim 1, whereinthe head has a stem trunion receiving cup in the generally planar end tothe head, and the stem has a trunion for being received in said cup. 14.The implant of claim 13, which has tapered walls to said cup and saidtrunion for securing the head and stem together.
 15. The implant ofclaim 13, wherein the head is made of a suitable ceramic material, andthe stem of a suitable metal.
 16. The implant of claim 1, wherein thehead is made of a suitable ceramic material, and the stem of a suitablemetal.
 17. The implant of claim 1, wherein the head has a 13-mm to 19-mmdiameter.
 18. A modular basal thumb joint implant comprising thefollowing parts: a head including a single, smooth, medio-proximallydirected, articulating surface, and a generally abrupt, distallydirected, planar end to the head which defines an end to saidarticulating surface, said articulating surface being continuous as toits sphericity and uninterrupted up to the end of said articulatingsurface so that said articulating surface defines a truncated ball of ashape that is greater than hemispherical; and a stem, which isattachable to the head, and which, when attached to the head, projectsfrom the head along an axis, which arises from the generally planar endto the head and includes the following features: A) a general angle ofjunction of the stem with the head that is about from sixty-five toseventy-five degrees in relation to the generally planar end to thehead; B) a tri-flanged cross-sectional stem profile; C) an inwardlycurved stem; and D) an eccentric head junction site for the stem—wherein said implant has its head of a size for mounting in andarticulating with a correspondingly concavely prepared surface oftrapezium bone stock, and its stem of a site for intramedullaryinsertion in metacarpal bone stock.
 19. The implant of claim 18, whereinthe general angle of the junction of the stem with the head is aboutseventy degrees in relation to the generally planar end to the head, andthe stem profile generally is T-shaped and tapers from the head to adistal end of the stem.
 20. In a basal thumb joint implant, whichincludes a head of a size and having an articular surface for mountingand articulating in association with a correspondingly concavelyprepared surface of trapezium bone stock, and a stem of a size forintramedullary insertion in metacarpal bone stock, the improvement whichcomprises head and stem modularity such that the head is removablyattachable to the stem.
 21. The improvement of claim 20, wherein thehead has a non-articular surface opposing the articular surface, and thehead is attachable to the stem with a general angle of projection of thestem from the head that is acute in relation to the non-articularsurface of the head.
 22. The improvement of claim 20, which includes forbone interface a porous coating.